This invention relates to a thermal transfer image receiving sheet for use in a thermal transfer system wherein a thermal transfer image receiving sheet is superposed on a thermal transfer sheet and a colorant is thermally transferred from the thermal transfer sheet onto the thermal transfer image receiving sheet to form an image on the thermal transfer image receiving sheet, and a process for producing the same. More particularly, the present invention relates to a thermal transfer image receiving sheet capable of preventing misregistration which is likely to occur in the formation of an image on a thermal transfer image receiving sheet, and a process for producing the same.
Formation of letters or images on an object by a thermal transfer system has hitherto been carried out in the art. A thermal dye transfer system (a thermal sublimation transfer system) and a thermal ink transfer system (a hot melt transfer system) have been extensively used as the thermal transfer system. The thermal dye transfer system is such that a sublimable dye is used as a colorant, and the dye in a sublimable dye layer provided in a thermal transfer sheet is thermally transferred onto an object, such as a thermal transfer image receiving sheet, by means of a heating device, such as a thermal head, wherein generation of heat is regulated according to image information, thereby forming an image on the object.
In the thermal dye transfer system, the amount of the dye transferred can be regulated for each dot by heating in very short time. The image thus formed is very sharp, because the colorant used is a dye, and, at the same time, is highly transparent. Therefore, the image has excellent halftone reproduction and gradation, high definition, and high quality comparable to that of full-color photographic images using a silver salt.
By virtue of development of various hardwares and softwares associated with multimedia, the thermal transfer system has found an expanded market as a full color hard copy system of digital images, including computer graphics, still pictures by satellite communication, and CDROM, and analog images, such as video images.
Specific applications of the thermal transfer image receiving sheet in the thermal transfer system are various, and representative examples thereof include proof printing, output of an image, output of a design, such as CAD/CAM, output applications for various medical instruments for analysis, such as CT scan and endoscope cameras, output applications for measuring equipment, alternatives for instant photography, output of photograph of a face to identification (ID) cards, credit cards, and other cards, and applications in composite photographs, pictures for keepsake, and picture postcards in amusement facilities, such as pleasure grounds, game centers, museums, aquariums, and the like.
One of the methods for forming an image in the thermal transfer system is to superimpose colors on top of each other or one another side by side. In this method, a color image is formed for each image face. Therefore, the thermal transfer image receiving sheet is reciprocated, and an image is transferred one color by one color from the thermal transfer sheet being successively wound in one direction to put colors on top of each other or one another. According to this method, advantageously, image printing speed is high, and there is no overlapping between lines experienced in the serial process. Therefore, a good image can be formed so far as the positioning accuracy is high. This method, however, suffers from problems including that, due to the nature of reciprocation of the thermal transfer image receiving sheet, the paper positioning accuracy is so low that the so-called misregistration is likely to occur, and, in addition, it is difficult to realize a reduction in size and cost of the image printer.
The following several printers have hitherto been proposed for use in the above method.
For example, one of them is a printer wherein the thermal transfer image receiving sheet is fastened in its one end with a chuck and in this state is reciprocated. In this printer, since the thermal transfer image receiving sheet is reciprocated using an independent chuck, the carrying accuracy is high. The printer can easily form an image on a thermal transfer image receiving sheet having a relatively large size, for example, a thermal transfer image receiving sheet of A3 size or larger. This printer, however, is disadvantageous in that the mechanism is complicated and, hence, the size of the printer should be increased, it is difficult to form an image on a thermal transfer image receiving sheet of small size, and the price of the printer is high.
Another printer is one wherein one end of the thermal transfer image receiving sheet is fixed to and wound on a chuck provided on the surface of a platen roller and reciprocated by rotation of the platen roller. Before carrying accuracy is discussed, this type of printer has a drawback that, when the thermal transfer image receiving sheet is delivered, it is likely to be jammed within the printer.
Still another printer is one wherein the thermal transfer image receiving sheet is fastened with a grip roller comprising a rubber roller and a metallic roller and reciprocated by utilizing the rotation of the roller. By virtue of a simple structure of this type of printer, the size thereof can be reduced, and the price of the printer is low. For this reason, at the present time, this printer has been most extensively used in the art.
In this printer, the grip roller comprises: a rubber roller for preventing the sheet from being slipped; and a metallic roller having fine protrusions having a height of about 40 to 100 xcexcm (hereinafter referred to as xe2x80x9cspikexe2x80x9d) formed thereon by etching that function to bite the thermal transfer image receiving sheet, thereby carrying the thermal transfer image receiving sheet with high accuracy. This grip roller, however, has originally been used mainly in monochrome printing apparatuses where the reciprocation of the thermal transfer image receiving sheet is not required, such as diazo copying of drawings and printers for drafting. Therefore, the carrying accuracy of a printer using this grip roller is not very good, and printing of an image while reciprocating the thermal transfer image receiving sheet has been likely to create misregistration. Increasing the pushing force of the rubber roller and the metallic roller can improve the carrying accuracy in the reciprocation of the thermal transfer image receiving sheet. In this case, however, the spike of the metallic roller bites the thermal transfer image receiving sheet, creating and leaving a trace of the spike. In particular, in the case of a thin thermal transfer image receiving sheet, the trace of the spike poses a serious problem. In some cases, the trace reaches the image receiving face on the surface of the thermal transfer image receiving sheet, resulting in deteriorated print quality.
In order to cope with the above problems, in view of properties of commercially available printers, for example, the carrying accuracy of the printer, the pushing force of the grip roller for improving the carrying accuracy, the degrees of the trace of spike created by increasing the pushing force of the grip roller and the like, thermal transfer image receiving sheets compatible with respective various printer types have hitherto been supplied. For example, the thickness of the thermal transfer image receiving sheet has been increased so that, even when the pushing force of the grip roller is increased to improve the carrying accuracy, the influence of the spike does not reach the image receiving face.
Supplying a wide variety of thermal transfer image receiving sheets compatible with respective various printers, however, results in remarkably increased development cost and production cost. Further, an increase in thickness of the thermal transfer image receiving sheet in order to reduce the influence of the spike limits the thickness and layer construction suitable for conventional thermal transfer image receiving sheets, often leading to restriction of functions commonly possessed by the thermal transfer image receiving sheet, for example, handle, nerve, gloss and other properties.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer image receiving sheet that, when used with various printers, can be carried with improved accuracy, can form an image without misregistration, can prevent a trace of a spike of a metallic roller in the printer from reaching the image receiving face and hence can form an image having quality not significantly influenced by the trace of the spike.
Another object of the present invention is to provide a process for producing a thermal transfer image receiving sheet that can easily produce a thermal transfer image receiving sheet which can form an image free from misregistration and an image free from a trace of a spike and having good quality.
According to one aspect of the present invention, there is provided a thermal transfer image receiving sheet comprising: a substrate; a receptive layer provided on at least one side of the substrate; and a grip layer provided on the other side of the substrate, the grip layer being constituted by an unstretched synthetic resin layer having a softening point of 110xc2x0 C. or above.
The grip layer is preferably an unstretched polyolefin resin layer or an unstretched polyester resin layer.
The unstretched polyolefin resin layer is preferably an unstretched polypropylene resin layer.
Preferably, a polyethylene resin layer is provided between the substrate and the grip layer.
Preferably, the grip layer is formed by extrusion lamination.
Preferably, the polyethylene resin layer and the grip layer are formed by coextrusion lamination.
Preferably, the grip layer has a thickness of 15 to 50 xcexcm.
According to another aspect of the present invention, there is provided a process for producing a thermal transfer image receiving sheet, comprising the steps of: forming a receptive layer on at least one side of a substrate; and extrusion-laminating a synthetic resin having a softening point of 110xc2x0 C. or above on the other side of the substrate to form a grip layer constituted by an unstretched synthetic resin layer.
Preferably, in the step of the extrusion lamination, a polyethylene resin and a synthetic resin having a softening point of 110 xc2x0 C. or above are coextrusion-laminated on at least one side of the substrate to prepare a thermal transfer image receiving sheet comprising a polyethylene resin layer and a grip layer constituted by an unstretched synthetic resin layer.
In the above process, the synthetic resin having a softening point of 110xc2x0 C. or above is preferably a polyolefin resin or a polyester resin.
In the above process, the polyolefin resin is preferably a polypropylene resin.
The thermal transfer image receiving sheet of the present invention comprises a substrate, a receptive layer provided on at least one side of the substrate, and a grip layer provided on the other side of the substrate. In this case, the grip layer is constituted by an unstretched synthetic resin layer having a softening point of 110xc2x0 C. or above. This constitution permits a spike of a metallic roller in a printer to satisfactorily bite the unstretched synthetic resin layer having a softening point of 110xc2x0 C. or above as the grip layer. This prevents slippage between the metallic roller and the thermal transfer image receiving sheet at the time of a reciprocation for image formation and hence can prevent misregistration. Therefore, the thermal transfer image receiving sheet can be carried with improved accuracy, and a thermally transferred image can be formed without misregistration.
In this case, the grip layer is preferably constituted by an unstretched polyolefin resin layer or an unstretched polyester resin layer, particularly preferably an unstretched polyolefin resin layer. In the unstretched polyolefin resin layer, an unstretched polypropylene resin layer is preferred.
The provision of a polyethylene resin layer between the substrate and the grip layer is advantageous in that the polyethylene resin layer can enhance the adhesion between the substrate and the grip layer. Further, the polyethylene resin layer can prevent a trace of a spike from extending from the backside and reaching the image receiving face.
Next, the process for producing a thermal transfer image receiving sheet according to the present invention comprises the steps of: a forming a receptive layer on at least one side of the substrate; and extrusion-laminating a synthetic resin having a softening point of 110xc2x0 C. or above on the other side of the substrate to form a grip layer constituted by an unstretched synthetic resin layer. The extrusion lamination of the synthetic resin having a softening point of 110xc2x0 C. or above permits a grip layer of a synthetic resin in an unstretched state to be easily formed on the other side of the substrate. This unstretched layer of the synthetic resin is easily bitten by the spike. Thus, the process of the present invention can easily form a thermal transfer image receiving sheet which can form a thermally transferred image without misregistration.
Further, formation of a thermal transfer image receiving sheet having a polyethylene resin layer and an unstretched synthetic resin layer as a grip layer by coextrusion lamination of a polyethylene resin and a synthetic resin having a softening point of 110xc2x0 C. or above on at least one side of the substrate is preferred. Since the polyethylene resin layer, which can improve the adhesion between the substrate and the grip layer and at the same time can prevent a trace of the spike from reaching the image receiving face, can be easily formed by coextrusion lamination together with the grip layer which can be well bitten by the spike. This can realize the formation of a thermally transferred image without misregistration and the formation of a thermally transferred image free from a trace of a spike and having good quality.